Power Saving Mode (PSM) and extended Discontinuous Reception (eDRX) are specified by the Third Generation Partnership Project (3GPP) as part of wireless networks' evolution to cater to Internet-of-Things (IoT) and machine-to-machine (M2M) requirements. PSM enables user equipment (UE) to enter a deep-sleep mode for long periods of time after notifying the base station. eDRX allows UEs to have longer sleep periods between a UE's paging checks. Such features reduce the amount of power associated with frequent communication requests normally sent by a base station. eDRX and PSM form a key part of the power-saving strategy for IoT devices, and provide important and significant improvements in terms of battery life for applications such as alarms, smart meters and other sensors, as well as lowering device costs.
When a UE is in a sleep period (e.g., for either PSM or eDRX), the UE cannot receive data or respond to page requests. Unless an application server (AS) is informed of a UE's active (or wake-up) window, when the AS attempts to send a short message (SM) or other data to a UE in PSM or eDRX, there will be a high likelihood that the UE is sleeping and unable to receive the SM or other data.